Various hydrofluorocarbons have been investigated in the industry as blowing agents for polyisocyanate based foams due to their low or nonexistent ozone depletion potentials. U.S. Pat. No. 4,997,706 discloses the use of closed cell rigid polyurethane foams blown with a C.sub.1 -C.sub.4 hydrofluorocarbon along with a blowing agent precursor such as water in amounts effective to lower the thermal conductivity of the foam relative to a similar foam made in the absence of the hydrofluorocarbon. Such rigid thermal insulation foams are generally poured or sprayed into a cavity or mold to make residential or commercial refrigeration cabinets, entry doors, or other applications where insulation is advantageous. The cavities into which the foaming mixture is poured or sprayed are often large and/or contain complex shapes which make it difficult for the foaming mixture to uniformly penetrate. In the case of a large cavity, the foam front begins to gel making it increasingly difficult for the mixture to cover the whole cavity and foam to a uniform density. The advantageous thermal insulation properties of the foam are defeated if gaps are left in the cavity where the foaming mixture could not penetrate, if bubbles form as a result of the foam shrinking due to poor dimensional stability, or if the density gradient is non-uniform which in turn leads to gaps or poor dimensional stability.
The flow characteristics of a foaming mixture becomes particularly critical when one employs a blowing agent which instantly volatilizes at atmospheric pressure and temperature, causing the foam to froth at the dispensing head. An example of such a blowing agent is 1,1,1,2-tetrafluoroethane (R-134a). A frothed foam has the consistency much like a shaving cream, which renders it difficult to evenly flow throughout a cavity.
When manufacturing a rigid closed cell polyisocyanate based foam in a cavity or pour in place application, the average hydroxyl number of the polyols are generally over 400 to increase the crosslinking density, provide structural strength, and prevent foam shrinkage. The higher the average hydroxyl number, however, the more isocyanate one must consume at an equivalent isocyanate index and the faster the ingredients react. It would be desirable to increase the formulation latitude and processing window by decreasing the average hydroxyl number of the polyols, with the attendant advantage of reducing the amount of isocyanate consume and slowing the reaction down to afford improved flow characteristics. Adding high levels of blow catalyst in an effort to improve flow actually impedes the flow and impairs foam properties because the high levels of catalyst too rapidly promote the formation of urethane foam matrix. Merely lowering the average hydroxyl number, however, typically results in sacrificing dimensional stability.
It would also be desirable to reduce the amount of hydrofluorocarbon, such as R-134(a), employed due to its cost, while retaining the same density of a foam blown with the original amount of the hydrofluorocarbon. Merely adding more water, however, tends to gel the foam quicker, which reduces the flow characteristics of the foaming mixture, degrades the dimensional stability of the foam, and causes the foam to have a coarse cell structure.